How should I handle a split in a retopology?
First step. Make flaps bending down between the legs.



Make a strip between them and make several loop cuts. You may have to experiment to find the correct number of loop cuts. This is important if you want to have an all quad model. The total number of edges around the opening must match the leg.



Round out the strip to match the correct shape.



Select the two loops and bridge them. If you have the correct number of loop cuts in the strip between the flaps, this should be all quads. Else you will get triangles.



Further adjust the geometry to make the bridged area more even.



Final result: